A battery disconnection system for a motor vehicle is known in which a detonator is used to cut an electrical lead connected to a terminal of the battery. The disadvantage of this system is that it is non-reversible--that is, after actuation, the electrical lead has to be replaced. GB-A-2260635 discloses a battery disconnection system which is mountable on a battery, but which does not provide protection from excessive currents or easy manual control.